Since the advent of the Industrial Revolution, mankind has consumed vast quantities of fossil fuels in creating modern society, at the same time increasing the concentration of carbon dioxide in the atmosphere. Man has also fostered a rise in carbon dioxide concentration through environmental destruction such as the destruction of forests.
Because global warming is caused by increases in greenhouse gases such as carbon dioxide, fluorocarbons and methane in the atmosphere, it is very important to reduce the atmospheric concentration of carbon dioxide, which contributes significantly to global warming. A variety of research on, for example, controlling carbon dioxide emissions and sequestering carbon dioxide is being carried out on a global scale.
Among such research, the polymerization reaction between carbon dioxide and epoxides discovered by Inoue et al. shows promise as a reaction that could help to address the problem of global warming, and is being actively investigated not only as a way of chemically fixing carbon dioxide, but also from the standpoint of utilizing carbon dioxide as a carbon resource (see Non-Patent Document 1).
Metal catalysts are generally used in polymerization reactions between carbon dioxide and epoxide. Such catalysts that have hitherto been disclosed include the product of a reaction between diethylzinc and a compound having a plurality of active hydrogens (see Non-Patent Document 2), zinc-containing solid catalysts obtained by directly contacting zinc oxide with an aliphatic dicarboxylic acid in mechanical grinding treatment means in the presence of an organic solvent (see Patent Document 1), organic salts of metals obtained by reacting a metal oxide such as zinc oxide or a metal hydroxide such as calcium hydroxide with a dicarboxylic acid such as isophthalic acid and a monocarboxylic acid such as propionic acid (see Patent Document 2), and zinc-containing solid catalysts obtained by reacting a zinc compound, an aliphatic dicarboxylic acid and a specific amount of an aliphatic monocarboxylic acid (see Patent Document 3).
However, when an aliphatic polycarbonate is produced in this way by a polymerization reaction between carbon dioxide and epoxide, residual metal catalyst remains within the resulting aliphatic polycarbonate. The stability over time or weathering resistance of secondary products produced using such a residual metal catalyst-containing aliphatic polycarbonate as the starting material may be compromised. To address this problem, methods of producing aliphatic polycarbonate which prevent beforehand the residual presence of metal catalyst have been disclosed, including, for example, a method of removing residual metal catalyst using an inorganic acid (see Non-Patent Document 3).
Patent Document 1: Japanese Kokai Publication Hei-2-47134 (JP-A Hei-2-47134)
Patent Document 2: Japanese Kokai Publication Sho-52-151116 (JP-A Sho-52-151116)
Patent Document 3: Japanese Kokai Publication 2007-302731 (JP-A 2007-302731)
Non-Patent Document 1: Macromolecular Syntheses, Vol. 7, p. 87 (1969)
Non-Patent Document 2: Kobunshi Ronbunshu (Collection of articles on polymer), Vol. 62, p. 131 (2005)
Non-Patent Document 3: Polymer Engineering and Science, Vol. 40, p. 1542 (2000)